The invention relates to fabrication of complex tubing structures which carry pressurized fluids. An example of such a structure is found in a fuel manifold in a gas turbine engine.
FIG. 1 is a partial schematic of a fuel manifold 3 used in a gas turbine engine (not shown). Tubes 6, which were originally straight when manufactured, are bent into the shapes shown using known procedures. The bent tubes 6 are connected to connectors 9.
Brazed joints (not shown) are often used to bond tubes 6 to connectors 9. If brazed joints are used, problems can arise which will be explained by reference to FIGS. 2-5.
FIG. 2 illustrates in schematic form a hollow tube 12. When the tube 12 is bent into the curved shape 15 shown in FIG. 3, the cross section becomes elliptical, as illustrated in exaggerated form by ellipse 18.
The deviation of ellipse 18 from a perfect circle is small: the major diameter 21 of the ellipse is greater than the minor diameter 24 by only a few percent, such as one or two percent. However, this small deviation from perfect circularity can create faulty braze joints.
For example, in FIG. 4, circle 14 represents a sleeve of a connector 9 in FIG. 1, and ellipse 16 represents tube 15 in FIG. 2. If the latter is inserted into the former, as in assembly 30, then contact may occur in region 36, because of a tight fit. The molten brazing alloy may not penetrate between the two components in region 36, causing a weak spot, and possible leakage. In addition, in order to create an ideal braze joint, a uniform distance 39 in FIG. 5 should exist between the two components 17 and 18 to be brazed. If one of the components is elliptical, as in FIG. 4, then this uniform distance 39 cannot be achieved.
Further, tube-to-tube variations further increase the non-uniformity. That is, a given bent tube 15 in FIG. 2 may fit tightly in region 36 in FIG. 4, while a second, supposedly identical, bent tube may not.
The inventor has developed a process for reducing the problems just described.
In one form of the invention, liquid is placed into a bent tube of elliptical cross-section. The liquid is pressurized to a pressure which causes the hoop stress in the tube to exceed the plastic deformation limit of the tube material. The tube is thereby deformed into a circular cross section.